The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as gas, oil, and water residing in a subterranean formation can be recovered by drilling wells into the formation. Well drilling involves drilling a wellbore down to the formation while circulating a drilling fluid through the wellbore. Various types of drilling fluids, also known as drilling muds, have been used in well drilling such as mineral oil-based fluids and synthetic oil-based fluids. Such drilling fluids typically form a thin, slick filter cake on the formation face that provides for successful drilling of the well bore and that helps prevent loss of fluid to the subterranean formation. In the hydrocarbon bearing portions of a formation, drilling fluids that produce filter cakes of cellulose and starch derivatives and sized calcium carbonate are often employed.
Several stages may be used to produce oil found in subterranean formations. The first stage, which is known as the primary production stage, allows the oil to flow into a production well (or wells) under natural forces. At first, the natural forces may be sufficient to drive the oil to the surface where it is recovered. However, at some point, pumps may be required to displace the oil from the wellbore to the surface. The primary production stage usually yields only about 5% to 15% of the oil in the reservoir. A secondary recovery operation thus is typically performed to recover additional amounts of the oil from the reservoir. A common secondary recovery operation known as secondary flooding involves injecting a fluid such as water into a so-called injection well (or wells) to drive oil in the formation to the production well (or wells). Secondary flooding usually recovers up to an additional 50% of the original oil in the reservoir. Tertiary recovery operations such as tertiary flooding may also be used to drive the remaining oil from the formation to the production well. Unfortunately, the presence of the filter cake on the face of the subterranean formation may adversely affect the flow of fluid though the injection and production wells. The filter cake may occlude the pore structure of the formation. In the case of the injection wells, particularly in deepwater environments, the injected fluid usually is not flowed back to remove the filter cake left by the drilling fluid. However, the pump pressures (e.g., fracturing pressures) required to inject past the filter cake may be higher than desirable for achieving good sweep efficiency of the oil.
A procedure known as acidization has been used for filter cake removal for over a century. In particular, the cellulose of which the filter cake is primarily composed may be decomposed by applying acid to the filter cake. It is believed that the first acidization procedure involved directly injecting strong mineral acids such as hydrochloric acid (HCl) into the well. However, the high reactivity of such strong acids commonly result in the rapid consumption of the acid before it can reach the desired treatment region where the filter cake was located. Further, such acids are highly corrosive and thus attack the metal parts of the well structure, causing irreversible damage to the well.
New acidization treatment solutions have been developed to overcome the problems associated with the use of mineral acids alone. For example, one such treatment solution includes hydrochloric acid emulsified in crude oil such that the aqueous phase, i.e. the solution of acid in water, is surrounded by a continuous oil phase emulsifier that prevents the acid from adversely affecting the metal parts of the well structure. See U.S. Pat. No. 1,922,154 to de Groote. A variation on this treatment solution uses a higher concentration of emulsifier to prolong the stability of the emulsion. See U.S. Pat. No. 2,050,932 to de Groote. Another treatment method involves removing any water in contact with the metal parts of the well before introducing HCl gas absorbed in a mineral oil that is practically immiscible with or insoluble in water to insulate the metal of the well from being attacked by the acid. See U.S Pat. No. 2,206,187 to Herbsman. Yet another method utilizes both an aqueous fluid and a non-aqueous fluid capable of forming or releasing an acid upon dilution with water. In particular, the well may be filled with oil to protect the metal from the acid, followed by pumping the aqueous fluid down to the formation. The non-aqueous fluid containing the acid-forming substance may then be introduced to the well. See U.S. Pat. No. 2,059,459 to Hund. A treatment solution that uses an ester, such as that derived from glycerol, as an emulsifying agent for an aqueous acid in oil is described in U.S. Pat. No. 2,681,889 to Menaul et al. The ester undergoes hydrolysis to break the emulsion and release the acid. A similar solution uses an acid anhydride such as acetic anhydride in a hydrocarbon carrier fluid to release acid upon reaction with water. See U.S. Pat. No. 2,863,832 to Perrine. A treatment solution comprising an anhydrous organic acid, such as formic acid, acetic acid, or propionic acid, in an anhydrous hydrocarbon has also been described in U.S. Pat. No. 2,910,436 to Alhambra et al. Unfortunately, such acids are as likely to be prematurely exhausted as mineral acids before reaching the desired treatment region. All of the above-mentioned patents are incorporated by reference herein.
One modern acidization method involves the generation of acids in the wellbore via the action of enzymes and suitable acid precursors. However, this method is limited by the heat tolerance of the particular enzyme being used and the breakdown temperature of the acid precursor. Treatment at high temperatures results in fast acid exhaustion and enzyme deactivation which results in poor filter cake removal. The enzymes and acid precursors thus need to be stored and handled at the well site carefully to avoid being exposed to relatively high temperatures due to heat and sunlight. Another method relies on the triggered release of acid via the lowering of the pH of an aqueous solution comprising an ortho ester to below about 7. Unfortunately, at elevated temperatures this release may occur in a relatively short period of time. Yet another method growing in popularity relies on the time-dependent reaction of certain esters, such as diethyleneglycol diformate, in an aqueous solution to generate acid such as formic acid. The esters currently being used for this purpose hydrolyze at relatively slow rates at temperatures less than 60° C. However, at higher temperatures those esters hydrolyze too quickly to allow the aqueous solution to be adequately dispersed across the entire filter cake before the acid is consumed. The filter cake removal thus may be localized to a proportionately small area when using such methods, further resulting in the premature loss of the acid-generating fluid through pores that have been unclogged by this localized removal. It is therefore desirable to develop an acidization method in which the acid-releasing reaction occurs at a relatively slow rate over a wide temperature range, particularly at relatively high temperatures.